During semiconductor wafer fabrication, light can be utilized in a lithographic process to enable transfer of very small lithographic patterns, such as nanometer-scale lithographic patterns, from a lithographic mask to a semiconductor wafer. In EUV lithography, for example, a laser-produced plasma (LPP) serves as a lithographic light source, at extreme ultraviolet (EUV) wavelengths. As a result of LPP light source emission, a pattern formed on a lithographic mask can be transferred to a semiconductor wafer by exposing a photoresist formed on the semiconductor wafer to the image of the lithographic mask.
A conventional method for producing LPP light emission in a lithographic system involves use of a focusing lens situated between the laser source and the target to focus the laser beam onto the target, thereby exciting the plasma. Focusing of the laser beam in a conventional method typically produces a distribution of laser beam intensities across the diameter of the target. Because the wavelength of light produced by an LPP light source can vary with the intensity of the laser beam incident on the target, conventional techniques providing a distribution of laser beam intensities at the target may result in the LPP light source emitting light in a distribution of wavelengths. In many situations, however, it is desirable to have an LPP light source provide a substantial portion of its emitted light at a specific wavelength, for example, 13.5 nanometers in an EUV lithographic process. Light that is significantly shorter or longer than the desired wavelength (out-of-band radiation) may not be useful to the lithographic process, and may have the undesirable effect of reducing image quality, and producing excess heat.